In a capital intensive field such as the production of polymers, it is imperative that the maximum amount of production be obtained from each piece of capital equipment. Therefore, when polymerization reactor trains for polyolefins are designed and installed, they are designed for the production of multiple products.
In particular, most polypropylene manufacturers produce both propylene homopolymers and copolymers, i.e. random or impact copolymers. An impact copolymer, is an in-situ blend of a propylene homopolymer with a copolymer of propylene and ethylene and/or other alpha-olefins. To produce such blends in-situ a reactor train having at least two stages is used, wherein the propylene homopolymer portion of the blend is produced in the first stage reactors and is then transferred to the second stage reactors where a copolymer of propylene with ethylene and/or other alpha-olefins is produced in the presence of the homopolymer.
Because of the high capital cost of equipment, the same reactor trains used to produce impact copolymers are used to produce propylene homopolymers. In this case, the second stage reactors are left idle and the homopolymer is recovered directly from the first stage reactors. However, keeping the second stage reactors idle during homopolymer production imposes its own cost, as reactor capacity is left unused for a time.
It would therefore be desirable to provide a method whereby historically unused reactor capacity can be brought into use for the production of propylene homopolymers. Such a process would provide a polymer product of uniform properties and quality.